Four types of commercially available prior art systems are used for applying polymeric resin powder coatings: 1) fluidized bed; 2) flame spray; 3) electrostatic; and 4) tribocharge (sometimes referred to as "tribo-electric"). Fluidized bed coating systems used for applying polymeric resin powder coatings are limited as to the location at which coating can be done and the size of the parts that can be coated. Flame spray, electrostatic and tribocharge applicator systems are more portable and typically comprise a powder source such as a hopper, an applicator device, or a conduit (usually a flexible hose) connecting the powder source to the applicator device.
In most conventional powder delivery systems, compressed air is forced through an eductor adjacent to the powder source. Powdered polymeric resin is entrained in the flow of compressed air and delivered through the hose to the applicator device.
More recently, a powder delivery system has been disclosed in U.S. Pat. No. 5,520,736, incorporated by reference herein, that is useful for supplying powder to a flame spray applicator device as disclosed in U.S. Pat. No. 5,297,733 that employs an eductor inside the applicator device to draw powder from the powder source. Flame spray powder delivery systems require the use of combustible gasses to heat resin particles prior to directing them against the substrate. With these systems, it is sometimes difficult to obtain a coating of even thickness, particularly if the substrate embodies complex surfaces.
Electrostatic and tribocharge applicator systems apply an electrical charge to the powder particles, thereby causing them to adhere to the substrate. Electrostatic powder delivery systems use an electrical transformer in or near the applicator device and a metal needle at the exit of the gun to create an ionizing field that applies a negative charge to the resin particles as they are carried by conveying air toward an electrically grounded substrate. Tribocharge coating systems utilize mechanical rubbing instead of a transformer to impart a static electrical charge to the polymeric resin particles. As powder particles are carried by air pressure through the applicator device, they rub against the interior wall of the spray gun applicator and the resultant friction creates a positive charge on the powdered particles. Electrostatic and tribocharge systems are discussed generally by William D. Lehr in Powder Coating Systems (McGraw-Hill, 1991), p. 96-99. Conventional prior art tribo-electric powder spray guns are disclosed in U.S. Pat. Nos. 3,903,321 and 5,402,940.
With regard to applicator devices incorporating a "swirler" in the applicator, U.S.S.R. Patent No. 1,162,502 discloses a flame spray gun having a "swirler" which contains on its biconical surface multi-start spiral grooves cut with an inclination of between 15 to 60 degrees from the central axis of the swirler. As the particles pass though the mixer, their flow is dictated by the grooves' which contributes to enhanced mixing. Therefore, the particles, upon exiting the gun, are more evenly distributed and more equally heated. FIGS. 6 through 8 of U.S. Pat. No. 5,282,573 illustrate a one direction spiral "bifurcating twisted vane" used in a conventional flame spray gun, wherein powdered resin is drawn from a hopper having an eductor in the hopper. The air carries or pushes the powdered resin through the supply hose to the applicator device as opposed to incorporating the eductor directly in the applicator device as is done in the present invention.